Body cavity fluids, such as ascitic fluid and pleural fluid, are concentrated and filtered to remove bacteria and cancer cells, and the filtered concentrate is returned to the patient intravenously. The concentration and filtration are performed batch-wise, with the concentration being done before the filtration. The apparatus for concentrating and filtering includes a first container for holding the body fluid, a second container for holding the final filtered concentrate, a filter, a concentrator, a pump and two branched tubes connected to the filter and the concentrator which tubes can be selectively closed for concentrating and filtering the body fluid. The fluid outlet of the first container is designed to help prevent precipitated fibrin from passing out of the container and blocking the concentrator or filter membranes.